sedimentary
Chert (Jasper/Agate variety)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale. Color: Brownish-red, tan, and waxy orange. Luster: Vitreous to waxy. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture).
- Hardness
- 6
- Color
- Brownish-red, tan, and waxy orange
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 6.5-7 on Mohs scale. Color: Brownish-red, tan, and waxy orange. Luster: Vitreous to waxy. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture).
Formation & geological history
Forms primarily as a biochemical or chemical sedimentary rock through the accumulation of silica-rich microfossils or precipitation from silica-rich fluids in limestone/dolomite host rocks. This specimen likely developed through groundwater precipitation within sedimentary cavities.
Uses & applications
Used historically for stone tools due to its sharp edges when fractured. Modern uses include decorative landscaping, lapidary work (cabochons), and as a minor abrasive.
Geological facts
Chert is so hard it can create sparks when struck against steel, which made it vital for flintlock firearms. Because of its durability, it often outlasts the host rock it formed in, leading to large 'nodules' found in riverbeds.
Field identification & locations
Identify in the field by its distinctive conchoidal (shell-like) fracture edges, waxy texture, and extreme hardness (it will scratch glass). Commonly found in sedimentary basins and as river-worn cobbles.
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